One method of measuring moisture content in powder is by means of near infrared spectral analysis in which the powder is irradiated with near infrared light, the light diffusely reflected from the powder is spectrally divided into its narrow band components, and then mathematically analyzed to provide an accurate measurement of moisture content. However, initial attempts to use near infrared spectral analysis to measure the moisture content of some powder, such as powdered milk, have been unsuccessful because the window through which the near infrared measurements were taken, quickly became coated with the powder and prevented transmission of infrared light between the powder and the instrument where the infrared light is to be analyzed.